diff --git a/firmware/application/apps/ui_flash_utility.cpp b/firmware/application/apps/ui_flash_utility.cpp
index a3658c3e..b2b30d76 100644
--- a/firmware/application/apps/ui_flash_utility.cpp
+++ b/firmware/application/apps/ui_flash_utility.cpp
@@ -30,6 +30,30 @@ static const char16_t* firmware_folder = u"/FIRMWARE";
 Thread* FlashUtilityView::thread{nullptr};
 static constexpr size_t max_filename_length = 26;
 
+bool valid_firmware_file(std::filesystem::path::string_type path) {
+    File firmware_file;
+    uint32_t read_buffer[128];
+    uint32_t checksum{1};
+
+    // test read of the whole file just to validate checksum (baseband flash code will re-read when flashing)
+    auto result = firmware_file.open(path.c_str());
+    if (!result.is_valid()) {
+        checksum = 0;
+        for (uint32_t i = FLASH_STARTING_ADDRESS; i < FLASH_ROM_SIZE / sizeof(read_buffer); i++) {
+            auto readResult = firmware_file.read(&read_buffer, sizeof(read_buffer));
+
+            // if file is smaller than 1MB, assume it's a downgrade to an old FW version and ignore the checksum
+            if ((!readResult) || (readResult.value() != sizeof(read_buffer))) {
+                checksum = FLASH_EXPECTED_CHECKSUM;
+                break;
+            }
+
+            checksum += simple_checksum((uint32_t)read_buffer, sizeof(read_buffer));
+        }
+    }
+    return (checksum == FLASH_EXPECTED_CHECKSUM);
+}
+
 FlashUtilityView::FlashUtilityView(NavigationView& nav)
     : nav_(nav) {
     add_children({&labels,
@@ -111,8 +135,11 @@ void FlashUtilityView::flash_firmware(std::filesystem::path::string_type path) {
     if (endsWith(path, u".tar")) {
         // extract, then update
         path = extract_tar(u'/' + path).native();
-        if (path.empty()) return;
     }
+
+    if (path.empty() || !valid_firmware_file(path.c_str()))
+        return;  // bad firmware image - just returning back to the file list
+
     ui::Painter painter;
     painter.fill_rectangle(
         {0, 0, portapack::display.width(), portapack::display.height()},
diff --git a/firmware/application/apps/ui_flash_utility.hpp b/firmware/application/apps/ui_flash_utility.hpp
index 01ea2875..00950b4e 100644
--- a/firmware/application/apps/ui_flash_utility.hpp
+++ b/firmware/application/apps/ui_flash_utility.hpp
@@ -32,8 +32,14 @@
 #include "untar.hpp"
 #include <cstdint>
 
+#define FLASH_ROM_SIZE 1048576
+#define FLASH_STARTING_ADDRESS 0x00000000
+#define FLASH_EXPECTED_CHECKSUM 0x00000000
+
 namespace ui {
 
+bool valid_firmware_file(std::filesystem::path::string_type path);
+
 class FlashUtilityView : public View {
    public:
     FlashUtilityView(NavigationView& nav);
diff --git a/firmware/application/ui_external_items_menu_loader.cpp b/firmware/application/ui_external_items_menu_loader.cpp
index f5b453d7..98fbe60e 100644
--- a/firmware/application/ui_external_items_menu_loader.cpp
+++ b/firmware/application/ui_external_items_menu_loader.cpp
@@ -107,6 +107,7 @@ namespace ui {
 
 /* static */ bool ExternalItemsMenuLoader::run_external_app(ui::NavigationView& nav, std::filesystem::path filePath) {
     File app;
+    uint32_t checksum{0};
 
     auto openError = app.open(filePath);
     if (openError)
@@ -115,7 +116,6 @@ namespace ui {
     application_information_t application_information = {};
 
     auto readResult = app.read(&application_information, sizeof(application_information_t));
-
     if (!readResult)
         return false;
 
@@ -135,6 +135,8 @@ namespace ui {
             if (!readResult)
                 return false;
 
+            checksum += simple_checksum((uint32_t)&application_information.memory_location[file_read_index], readResult.value());
+
             if (readResult.value() < std::filesystem::max_file_block_size)
                 break;
         }
@@ -156,6 +158,8 @@ namespace ui {
             if (!readResult)
                 return false;
 
+            checksum += simple_checksum((uint32_t)target_memory, readResult.value());
+
             if (readResult.value() != bytes_to_read)
                 break;
         }
@@ -168,11 +172,16 @@ namespace ui {
             if (!readResult)
                 return false;
 
+            checksum += simple_checksum((uint32_t)&application_information.memory_location[file_read_index], readResult.value());
+
             if (readResult.value() < std::filesystem::max_file_block_size)
                 break;
         }
     }
 
+    if (checksum != EXT_APP_EXPECTED_CHECKSUM)
+        return false;
+
     application_information.externalAppEntry(nav);
     return true;
 }
diff --git a/firmware/application/ui_external_items_menu_loader.hpp b/firmware/application/ui_external_items_menu_loader.hpp
index b4345b9e..0c3b04c8 100644
--- a/firmware/application/ui_external_items_menu_loader.hpp
+++ b/firmware/application/ui_external_items_menu_loader.hpp
@@ -29,6 +29,8 @@
 
 #include "file.hpp"
 
+#define EXT_APP_EXPECTED_CHECKSUM 0x00000000
+
 namespace ui {
 
 template <size_t Width, size_t Height>
diff --git a/firmware/application/ui_navigation.cpp b/firmware/application/ui_navigation.cpp
index 1e442990..d3ebe0e9 100644
--- a/firmware/application/ui_navigation.cpp
+++ b/firmware/application/ui_navigation.cpp
@@ -503,8 +503,8 @@ void SystemStatusView::rtc_battery_workaround() {
             month = (timestamp.FAT_date >> 5) & 0xF;
             day = timestamp.FAT_date & 0x1F;
 
-            // bump to next month at 28 days for simplicity
-            if (++day > 28) {
+            // bump to next month
+            if (++day > rtc_time::days_per_month(year, month)) {
                 day = 1;
                 if (++month > 12) {
                     month = 1;
@@ -547,16 +547,16 @@ InformationView::InformationView(
                   &ltime});
 
 #if GCC_VERSION_MISMATCH
-    static constexpr Style style_gcc_warning{
-        .font = font::fixed_8x16,
-        .background = {33, 33, 33},
-        .foreground = Color::yellow(),
-    };
-    version.set_style(&style_gcc_warning);
+    version.set_style(&Styles::yellow);
 #else
     version.set_style(&style_infobar);
 #endif
 
+    if (firmware_checksum_error()) {
+        version.set("FLASH ERROR");
+        version.set_style(&Styles::red);
+    }
+
     ltime.set_style(&style_infobar);
     refresh();
     set_dirty();
@@ -568,6 +568,17 @@ void InformationView::refresh() {
     ltime.set_date_enabled(pmem::clock_with_date());
 }
 
+bool InformationView::firmware_checksum_error() {
+    static bool fw_checksum_checked{false};
+    static bool fw_checksum_error{false};
+
+    // only checking firmware checksum once per boot
+    if (!fw_checksum_checked) {
+        fw_checksum_error = (simple_checksum(FLASH_STARTING_ADDRESS, FLASH_ROM_SIZE) != FLASH_EXPECTED_CHECKSUM);
+    }
+    return fw_checksum_error;
+}
+
 /* Navigation ************************************************************/
 
 bool NavigationView::is_top() const {
diff --git a/firmware/application/ui_navigation.hpp b/firmware/application/ui_navigation.hpp
index 61333e15..4b20cc82 100644
--- a/firmware/application/ui_navigation.hpp
+++ b/firmware/application/ui_navigation.hpp
@@ -286,6 +286,7 @@ class InformationView : public View {
    public:
     InformationView(NavigationView& nav);
     void refresh();
+    bool firmware_checksum_error();
 
    private:
     // static constexpr auto version_string = "v1.4.4"; // This is commented out as we are now setting the version via ENV (VERSION_STRING=v1.0.0)
diff --git a/firmware/application/usb_serial_shell.cpp b/firmware/application/usb_serial_shell.cpp
index 26fdf118..96a220db 100644
--- a/firmware/application/usb_serial_shell.cpp
+++ b/firmware/application/usb_serial_shell.cpp
@@ -41,6 +41,7 @@
 #include "chprintf.h"
 #include "chqueues.h"
 #include "ui_external_items_menu_loader.hpp"
+#include "ui_flash_utility.hpp"
 #include "untar.hpp"
 #include "ui_widget.hpp"
 
@@ -170,7 +171,13 @@ static void cmd_flash(BaseSequentialStream* chp, int argc, char* argv[]) {
     } else if (strEndsWith(path.native(), u".bin")) {
         // nothing to do for this case yet.
     } else {
-        chprintf(chp, "error only .bin or .ppfw.tar files canbe flashed.\r\n");
+        chprintf(chp, "error only .bin or .ppfw.tar files can be flashed.\r\n");
+        nav->pop();
+        return;
+    }
+
+    if (!ui::valid_firmware_file(path.native().c_str())) {
+        chprintf(chp, "error corrupt firmware file.\r\n");
         nav->pop();
         return;
     }
diff --git a/firmware/common/portapack_persistent_memory.cpp b/firmware/common/portapack_persistent_memory.cpp
index 7723f77c..3d711462 100644
--- a/firmware/common/portapack_persistent_memory.cpp
+++ b/firmware/common/portapack_persistent_memory.cpp
@@ -32,6 +32,7 @@
 #include "string_format.hpp"
 #include "ui_styles.hpp"
 #include "ui_painter.hpp"
+#include "ui_flash_utility.hpp"
 #include "utility.hpp"
 #include "rtc_time.hpp"
 
@@ -1051,6 +1052,9 @@ bool debug_dump() {
     pmem_dump_file.write_line("Ext APPS version req'd: 0x" + to_string_hex(VERSION_MD5));
     pmem_dump_file.write_line("GCC version: " + to_string_dec_int(__GNUC__) + "." + to_string_dec_int(__GNUC_MINOR__) + "." + to_string_dec_int(__GNUC_PATCHLEVEL__));
 
+    // firmware checksum
+    pmem_dump_file.write_line("Firmware calculated checksum: 0x" + to_string_hex(simple_checksum(FLASH_STARTING_ADDRESS, FLASH_ROM_SIZE), 8));
+
     // write persistent memory
     pmem_dump_file.write_line("\n[Persistent Memory]");
 
diff --git a/firmware/common/utility.cpp b/firmware/common/utility.cpp
index d4cbdc26..36446e0a 100644
--- a/firmware/common/utility.cpp
+++ b/firmware/common/utility.cpp
@@ -235,3 +235,10 @@ std::string join(char c, std::initializer_list<std::string_view> strings) {
 
     return result;
 }
+
+uint32_t simple_checksum(uint32_t buffer_address, uint32_t length) {
+    uint32_t checksum = 0;
+    for (uint32_t i = 0; i < length; i += 4)
+        checksum += *(uint32_t*)(buffer_address + i);
+    return checksum;
+}
\ No newline at end of file
diff --git a/firmware/common/utility.hpp b/firmware/common/utility.hpp
index b1688caf..924297f6 100644
--- a/firmware/common/utility.hpp
+++ b/firmware/common/utility.hpp
@@ -217,4 +217,6 @@ struct range_t {
 
 std::string join(char c, std::initializer_list<std::string_view> strings);
 
+uint32_t simple_checksum(uint32_t buffer_address, uint32_t length);
+
 #endif /*__UTILITY_H__*/
diff --git a/firmware/tools/export_external_apps.py b/firmware/tools/export_external_apps.py
index 228d82e6..8249ab9e 100755
--- a/firmware/tools/export_external_apps.py
+++ b/firmware/tools/export_external_apps.py
@@ -104,6 +104,14 @@ for external_image_prefix in sys.argv[4:]:
 		external_application_image = patch_image(external_application_image, search_address, replace_address)
 		external_application_image[memory_location_header_position:memory_location_header_position+4] = replace_address.to_bytes(4, byteorder='little')
 
+		checksum = 0
+		for i in range(0, len(external_application_image), 4):
+			checksum += external_application_image[i] + (external_application_image[i + 1] << 8) + (external_application_image[i + 2] << 16) + (external_application_image[i + 3] << 24)
+
+		final_checksum = 0
+		checksum = (final_checksum - checksum) & 0xFFFFFFFF
+		external_application_image += checksum.to_bytes(4, 'little')
+
 		write_image(external_application_image, "{}/{}.ppma".format(binary_dir, external_image_prefix))
 		continue
 
@@ -127,5 +135,13 @@ for external_image_prefix in sys.argv[4:]:
 		print("application {} can not exceed 32kb: {} bytes used".format(external_image_prefix, len(external_application_image)))
 		sys.exit(-1)
 
+	checksum = 0
+	for i in range(0, len(external_application_image), 4):
+		checksum += external_application_image[i] + (external_application_image[i + 1] << 8) + (external_application_image[i + 2] << 16) + (external_application_image[i + 3] << 24)
+
+	final_checksum = 0
+	checksum = (final_checksum - checksum) & 0xFFFFFFFF
+	external_application_image += checksum.to_bytes(4, 'little')
+
 	# write .ppma (portapack mayhem application)
 	write_image(external_application_image, "{}/{}.ppma".format(binary_dir, external_image_prefix))
diff --git a/firmware/tools/make_spi_image.py b/firmware/tools/make_spi_image.py
index a7151793..812028b3 100755
--- a/firmware/tools/make_spi_image.py
+++ b/firmware/tools/make_spi_image.py
@@ -74,7 +74,24 @@ for image in images:
 	padded_data = image['data'] + (spi_image_default_byte * pad_size)
 	spi_image += padded_data
 
-if len(spi_image) > spi_size:
-	raise RuntimeError('SPI flash image size of %d exceeds device size of %d bytes' % (len(spi_image), spi_size))
+if len(spi_image) > spi_size - 4:
+	raise RuntimeError('SPI flash image size of %d exceeds device size of %d bytes' % (len(spi_image) + 4, spi_size))
+
+pad_size = spi_size - 4 - len(spi_image)
+for i in range(pad_size):
+	spi_image += spi_image_default_byte
+
+# quick "add up the words" checksum:
+checksum = 0
+for i in range(0, len(spi_image), 4):
+	checksum += (spi_image[i] + (spi_image[i + 1] << 8) + (spi_image[i + 2] << 16) + (spi_image[i + 3] << 24))
+
+final_checksum = 0
+checksum = (final_checksum - checksum) & 0xFFFFFFFF
+
+spi_image += checksum.to_bytes(4, 'little')
 
 write_image(spi_image, output_path)
+
+percent_remaining = round(1000 * pad_size / spi_size) / 10;
+print ("Space remaining in flash ROM:", pad_size, "bytes (", percent_remaining, "%)")
\ No newline at end of file
diff --git a/firmware/tools/simple_checksum.py b/firmware/tools/simple_checksum.py
new file mode 100644
index 00000000..f3e9f9d1
--- /dev/null
+++ b/firmware/tools/simple_checksum.py
@@ -0,0 +1,57 @@
+#!/usr/bin/env python3
+
+#
+# Copyright (C) 2024 Mark Thompson
+#
+# This file is part of PortaPack.
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 2, or (at your option)
+# any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program; see the file COPYING.  If not, write to
+# the Free Software Foundation, Inc., 51 Franklin Street,
+# Boston, MA 02110-1301, USA.
+#
+
+import sys
+import struct
+
+usage_message = """
+PortaPack ROM image checksum checker
+
+Usage: <command> <input-file>
+"""
+
+def read_image(path):
+	f = open(path, 'rb')
+	data = f.read()
+	f.close()
+	return data
+
+def write_image(data, path):
+	f = open(path, 'wb')
+	f.write(data)
+	f.close()
+
+if len(sys.argv) != 2:
+	print(usage_message)
+	sys.exit(-1)
+
+image = read_image(sys.argv[1])
+image = bytearray(image)
+
+# simple "add up the words" checksum:
+checksum = 0
+for i in range(0, len(image), 4):
+	checksum += (image[i] + (image[i + 1] << 8) + (image[i + 2] << 16) + (image[i + 3] << 24))
+
+checksum &= 0xFFFFFFFF
+print ("Simple checksum =", checksum)